Offshore platforms, such as offshore oil rigs, include a motion compensation system that utilizes large hydraulic cylinders to stabilize the platforms. The hydraulic cylinders include rods, which are susceptible to corrosion caused by exposure to the elements. These large hydraulic cylinders also are used in civil engineering and hydropower applications including locks, dams and bridges.
The rods of the hydraulic cylinders may include solid or tubular rods manufactured from an austenitic nickel-chromium based alloy or a duplex stainless steel. However, the rods manufactured from the nickel-chromium based alloy or the duplex stainless steel are expensive. An alternative includes constructing the rods with a metallic cladding welded to an exterior surface of a rod machined from conventional steel. The metallic cladding protects the rods from corrosion, provides wear resistance and is a lower cost alternative to the solid or tubular rods manufactured entirely from the nickel-chromium based alloy or the duplex stainless steel.
The rods having the metallic cladding are typically manufactured from SAE 4130 steel. The protective metallic cladding may include a stainless steel material, a nickel based material or a cobalt based material. The metallic claddings may be deposited onto the rod by one of a number of fusion welding processes, including a laser welding process, a Plasma Transferred Arc (PTA) welding process, a Submerged Arc Welding (SAW) process, a Gas Tungsten Arc Welding (GTAW) process, a Gas Metal Arc Welding (GMAW) process, and a Shielded Metal Arc Welding (SMAW) process. Common to all of the above welding processes is the melting of the substrate, i.e., the outer surface of the rod, and the deposition of the metallic cladding material.
If the metallic cladding is not properly deposited onto the rod, then the metallic cladding may include a discontinuity in the metallic cladding, i.e., a defect in the metallic cladding. The discontinuities of the metallic cladding may include porosity in the metallic cladding, cluster porosity in the metallic cladding, cracks in the metallic cladding, shrink of the metallic cladding, incomplete fusion of the metallic cladding onto the rod, excessive penetration (high dilution) of the metallic cladding onto the rod or disbanding of the metallic cladding from the rod.
Additionally, discontinuities in the metallic cladding may form during use of the hydraulic rod in service. For example, small surface and subsurface discontinuities that would be barely noticeable at the time of weld cladding, can propagate due to high bending stresses, impacts and fatigue loads. Very high reversed bending loads can initiate and propagate fatigue cracks in the metallic cladding from the most minute stress risers like oxidized grain boundaries.
The service life of the rod is further complicated by the harsh marine environment in which the hydraulic cylinders must operate. Water borne debris, including ice, may impact the rods. Splashing water continually replenishes oxygen within surface opening discontinuities. Propagating discontinuities, such as pores and cracks in the metallic cladding, may link up to provide a pathway to the bondline between the surface of the rod and the metallic cladding. Once water reaches the steel of the rod at the bondline, corrosion occurs rapidly. Bondline corrosion eventually causes the metallic cladding to spall off, leaving the steel of the rod exposed and unprotected. Eventually, the propagation of the discontinuities in the metallic cladding, corrosion of the steel rod and spalling of the metallic cladding critically damage the rod, and may cut or tear seals and may result in the failure of the cylinder.